SIP vulnerablities

I want to prove SIP vulnerabilities using SiVuS (The VoIp vulnerability scanner ) tool.
The test Bed contains Asterisk server and two xlite clients. When Two xlite clients are registered with server. I am able to Remove their registrations from server by generating REGISTER SIP messages using SiVuS from another computer by keeping expiration time ‘0’. But i am not able to send BYE messages while call is going on between two clients in order to terminate the call. i am not successful at this attack.Can some one help me through this.
Thank you
Regards
Sindhu